The present invention relates to a gang control-rod controlling system for simultaneously operating a plurality of control rods, and more particularly to a gang control-rod controlling system and a reactor operation method, which are effective in exchange of a control rod pattern in boiling water reactors.
In boiling water reactors, several hundreds of fuel assemblies are accommodated in the reactor and one control rod for controlling reactor power is installed for every four fuel assemblies. In existing boiling water reactors, control rods are operated one by one. More specifically, first, by actuating a control rod select switch, the radial position of a control rod to be operated is selected. Then, by depressing an "insert" or "withdraw" button, the control is inserted or withdrawn to a predetermined axial position.
At the start-up of a reactor, reactor power is increased by withdrawing control rods. At this time, it is required to surely restrict a procedure of operating the control rods within a certain allowable limit so that reactivity worth of each control rod will not be too large. The reason of holding the reactivity worth of each control rod low is as follows. Should there occurs such an accident that any control rod is continuously withdrawn by mistake or that any control rod is dislodged to slip off from the reactor, the amount of radioactive material discharged due to damage of fuel assemblies could be kept within an allowable range from the standpoint of safety evaluation if the reactivity worth applied upon such an event is held low. A rod worth minimizer system (hereinafter referred to as an RWM system or simply as RWM) is known as means for preventing control rods from being withdrawn departing from a predetermined control rod pattern.
More specifically, such an RWM system functions to monitor respective positions of control rods and, should an operator attempts to select or withdraw the control rod deviating from a predetermined sequence of control rod operations, to issue an alarm or prevent the attempted operation. However, when reactor power is continuously raised in conformity with the RWM rules, monitoring by the RWM is no longer necessary over a certain power level because the reactivity worth of any control rods becomes small regardless of which control rod is selected. That power level at which the RWM is to be released is usually set to 10 % -35 % of the rated power. In other words, the operation of withdrawing control rods in the range below the power level at which the RWM is to be released must follow the predetermined sequence of control rod operations. Note that JP, A, 49-89094 is known as a prior patent relating to the RWM.
The procedure of withdrawing control rods that can hold the reactivity worth of each control rod small is basically to select the control rods to be withdrawn in such order that they are evenly distributed in the radial direction of a core, and also not to successively withdraw those control rods which are adjacent to each other. If two control rods adjacent to each other are withdrawn in succession, the power density at that position would be so extremely increased that the reactivity worth of those control rods becomes too large.
Two A type and B type sequences are known as the procedure of withdrawing control rods that can follow the RWM rules. The A type sequence is utilized to configure an A type control rod pattern and the B type sequence is utilized to configure a B type control rod pattern. The term "A type control rod pattern" is here used to mean a pattern of only those control rods arranged in the form of a checker board including a control rod at the core center. The term "B type control rod pattern" is here used to mean a pattern of only those control rods arranged in the form of a checker board in which a control rods at the core center is not included.
The A type sequence is set such that the control rods (the B-group control rods) of about half the number of total control rods arranged in the form of a checker board in which a center control rod is not included are divided into four groups, i.e., groups 1 to 4, in such a manner that the control rods of the respective groups are evened in number and arrangement in order to follow the RWM rules so that the reactivity worth of any withdrawn control rod may be held below a certain reference value, and the remaining about half control rods (the A-group control rods) are divided into 18 groups, i.e., groups 5 to 22, each comprising one, four, eight or twelve control rods, in consideration of symmetry. Also, the B type sequence is set such that the control rods (the A-group control rods) of about half the number of total control rods arranged in the form of a checker board including a center control rod are divided into four groups, i.e., groups 1 to 4, in such a manner that the control rods of the respective groups are evened in number and arrangement in order to follow the RWM rules so that the reactivity worth of any withdrawn control rod may be held below a certain reference value, and the remaining about half control rods (the A-group control rods) are divided into 18 groups, i.e., groups 5 to 22, each comprising two, four, eight or twelve control rods, in consideration of symmetry.
In the case of configuring the A type control rod pattern by using the A type sequence, to follow the RWM rules, the control rods of the groups 1 to 4 are first operated one by one to be fully withdrawn. As a result, about half the control rods in the entire core are withdrawn in the form of a checker board. This means that whichever control rod is selected at the time of subsequently withdrawing any control rod in the groups 5 to 22, all the control rods adjacent thereto have already been withdrawn and, therefore, the reactivity worth of each control rod is held small. In the above process, the control rods of the same group are always operated to position at the same axial level. Specifically, when some one control rod is withdrawn to a certain axial position, any control rod belonging to other groups shall not be withdrawn until all the remaining control rods of the same group are withdrawn to the same axial position. Also, the B type control rod pattern is configured in a like manner by using the B type sequence.
In practical use, only one of the A type sequence and the B type sequence is usually stored in a storage of a central processing unit. Thus, the A type sequence is stored in the storage when the A type control rod pattern is to be configured, and the B type sequence is stored in the storage instead of the A type sequence when the B type control rod pattern is to be configured.
While in existing boiling water reactors control rods are withdrawn one by one in conformity with the RWM rules as stated above, gang operation of withdrawing several control rods has been proposed in recent years. Adopting such gang operation for control rods enables cut-down of a time required for the control rod operations and hence a start-up time. During the gang operation, it is also required to follow the RWM rules in the stage of low power. Accordingly, gang control-rod groups in each of which control rods are operated at the same time must be defined in such a manner as able to follow the RWM rules.
When the gang operation is adopted to simultaneously withdraw several control rods, it is assumed that those control rods which are simultaneously withdrawn belong to the same control group. In the case of configuring the A type control rod pattern, the aforesaid A type sequence is utilized to select the control rod group to follow the RWM rules as with the existing scheme to operate control rods one by one. In the A type sequence, the number of control rods for each of the groups 1 to 4 is about 1/8 of the total number of control rods in the core. By setting the number of ganged control rods in the groups 1 to 4 so large, the start-up time can be cut down. Specifically, the ganged control rods of the groups 1 to 4 are first fully withdrawn group by group. After that, the ganged control rods of the groups 5 to 22 are withdrawn group by group in view of power distribution across the core since the reactor power is increased. As a result, the A type pattern is configured as a final objective pattern. Also, in the case of configuring the B type control rod pattern, the aforesaid B type sequence is utilized in a like manner.
Meanwhile, reactors usually keep on operating for approximately one year, but the control rod pattern is required to be exchanged several times during the continued operation. Let it now be supposed that the control rod pattern is exchanged from the A type to the B type. In this control rod pattern exchange, the control rods used in the B type pattern is first inserted to lower the power level, and the control rods used in the A type pattern is then withdrawn to raise the power level. On this occasion, the steps of inserting and withdrawing the control rods are not carried out at a time, but repeated several times so that the reactor power will not be extremely decreased. In other words, that step of operating the control rods is performed above the power level at which the RWM is to be released.
In the existing scheme, since the control rods are operated one by one above the power level at which the RWM is to be released, the aforesaid control rod pattern exchange is carried out by first indicating the radial control rod position to select the control rod to be operated, and then actuating an "insert" or "withdraw" button. However, the following problem arises when the gang operation of control rods is adopted.
When inserting the B type pattern control rods, the B type sequence must be selected as a sequence of control rod operations rather than the A type sequence. The reason is that if the A type sequence is selected, even those control rods which must be kept fully withdrawn would be inserted through the gang operation whichever one of the control rod groups 1 to 4 is selected. To the contrary, when withdrawing the A type pattern control rods, the A type sequence must be selected for the same reason. Thus, it is required during exchange of the control rod pattern to repeat several times the steps of inserting the A type pattern control rods and withdrawing the B type pattern control rods, which necessitates one of the B type sequence and the A type sequence to be selected for each of the steps.
In the above process, the A type sequence is practically selected by storing the A type sequence in the storage of the central processing unit, and the B type sequence is also selected by storing the B type sequence in the storage. This means that each time the other sequence is selected, the sequence currently stored in the storage requires to be changed, resulting in the very complicated operation.
On the other hand, control rods are quite important as means for controlling reactivity of reactors and required to have high reliability in operation. This necessitates that in the gang control-rod operation to operate a plurality of control rods at the same time, only those control rods which are designated as belonging to the same group are surely operated at the same time. Whenever the stored sequence is changed, therefore, it is indispensable to confirm whether the newly stored sequence is correct, or whether any error has occurred. Alternate selection of the A type sequence and the B type sequence entails confirmation of the newly stored sequence whenever it is stored, which makes the operation more complicated and deteriorates the reliability.